1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having circuits that are composed of thin film transistors (hereinafter referred to as TFTs). Specifically, the invention relates to the structure of electro-optical devices represented by liquid crystal display devices and of electric appliances having as their parts the electro-optical devices, and the invention also relates to how to manufacture the devices. The term semiconductor device herein refers to a device in general which utilizes semiconductor characteristics to function, and the electro-optical devices and electric appliances described above fall within this category.
2. Description of the Related Art
A technique that has been a popular research subject in recent years is to use laser annealing to crystallize an amorphous semiconductor film formed on an insulating substrate such as a glass substrate or to improve crystallinity of a crystallized film. The amorphous semiconductor film is often formed from silicon.
A glass substrate has advantages over a synthesized quartz glass substrate often used in the past, for it is inexpensive, is readily processible, and easily allows a large surface area to be obtained. These are the reasons for the flood of researches mentioned above. Laser annealing is preferred in crystallizing a film on a glass substrate because glass substrates have low melting point. A laser can give high energy only to an amorphous semiconductor film without increasing the temperature of the glass substrate on which the film is formed much.
A crystalline semiconductor is composed of many crystal grains and hence also called a polycrystalline semiconductor. A semiconductor film having crystal grains whose grain size is larger than the grain size of crystal grains of a semiconductor film is called a crystalline semiconductor film. A crystalline semiconductor film formed by laser annealing has high mobility. Therefore TFTs formed from crystalline semiconductor films are frequently used in, for example, a monolithic liquid crystal electro-optical device in which pixel TFTs and driver circuit TFTs are formed on the same glass substrate.
An annealing method that is highly productive and industrially superior and hence is widely employed includes: choosing a high power pulse laser such as an excimer laser; processing the pulse laser beam by an optical system into a spot beam that forms a few centimeter square on an irradiation surface, or into a linear beam extending 10 centimeters or longer on the irradiation surface; and performing scanning with the processed laser beam over the irradiation surface (or moving the laser beam irradiation position relative to the irradiation surface).
The linear laser beam is particularly productive, for laser irradiation of the entire irradiation surface can be done by running the linear beam only in the direction perpendicular to the longitudinal direction of the linear beams, unlike the spot-like laser beam that has to be used for scanning in both longitudinal and lateral directions. The linear laser beam is run in the direction perpendicular to the longitudinal direction because it is the most efficient scanning direction. Owing to this high productivity, laser annealing that uses a linear beam obtained by processing a pulse oscillation excimer laser beam through an appropriate optical system is becoming a mainstream technique for manufacturing a liquid crystal display device or the like from TFTs. This technique has made a monolithic liquid crystal display device reality in which TFTs for forming a pixel portion (pixel TFTs) and TFTs for forming driver circuits to be provided in the periphery of the pixel portion are formed on the same glass substrate.
However, in a crystalline semiconductor film formed by laser annealing, plural crystal grains mass, so that the crystal grains with irregular grain sizes are distributed unevenly. In a TFT formed on a glass substrate, its crystalline semiconductor film is divided into island-like patterns in order to separate elements. With crystal grains of irregular grain sizes distributed unevenly, it is impossible to specify the position and the size of the crystal grains in forming a TFT. There are much more recombination centers and trap centers due to the amorphous structure or crystal defects in the interface between crystal grains (crystal grain boundary) than inside the crystal grains. It is known that if carriers are trapped in these trap centers, the potential in the crystal grain boundary is raised to block the carriers and degrade the current transportation characteristic of the carriers. While electric characteristics of a TFT heavily depend on the crystallinity of the semiconductor film for forming a channel formation region; it has been almost impossible to remove the adverse effects of crystal grain boundary and form the channel formation region from a single crystal semiconductor film.
In order to solve those problems, various attempts have been made to control the position of crystal grains and increase the grain size by laser annealing. Now, a process a semiconductor film takes to solidify after the semiconductor film is irradiated with a laser beam is described first.
It takes a while for the semiconductor film that has been thoroughly melted by laser beam irradiation to form crystal nuclei. When an infinite number of crystal nuclei are evenly (or unevenly) generated in a thoroughly melted region and grow into crystals, the solidification process is completed for the thoroughly melted semiconductor film. The crystal grains obtained through this are distributed unevenly and have irregular grain sizes.
If the laser beam irradiation fails to melt the semiconductor film thoroughly and a solid phase semiconductor region partially remains, crystal growth is started immediately after the laser beam irradiation from the solid phase semiconductor regions. As mentioned before, it takes a while for the thoroughly melted region to generate crystal nuclei. Therefore, until crystal nuclei are generated in the thoroughly melted region, solid-liquid interface (meaning the border between the solid phase semiconductor region and the thoroughly melted region) that is the crystal growth front moves in a direction parallel to the surface of the semiconductor film (hereinafter referred to as lateral direction). This causes crystal grains to grow to gain a length several tens longer than the thickness of the semiconductor film. Such growth is ended when an infinite number of crystal nuclei are evenly (or unevenly) generated and grow into crystals in the thoroughly melted region. This phenomenon will hereinafter be called a super lateral growth.
An amorphous semiconductor film and a polycrystalline semiconductor film also have a region in which the energy of the laser beam is high enough to induce the super lateral growth. However, such high energy region is very narrow and where a crystal grain having a large grain size is to be formed cannot be controlled. In addition, regions other than the region in which crystal grains having a large grain size are formed are microcrystalline regions in which an infinite number of crystal nuclei are generated, or amorphous regions.
As described above, the position and the direction of crystal grain growth can be controlled if the temperature gradient in the lateral direction can be controlled (namely, if a heat flow running in the lateral direction can be generated) in the high energy region in which the energy of a laser beam is high enough to melt the semiconductor film thoroughly. Achieving this control has been tackled from various angles.
For example, a method of forming crystal grains at designed positions is described in “Lateral growth control in excimer laser crystallized polysilicon: Thin Solid Films 337 (1999), p 137-p 142). First, a metal film (a single layer of Cr or a laminate film obtained by layering an Al film on a Cr film) is formed on an amorphous semiconductor film and is partially etched to form a metal film region and a metal film less region on the amorphous semiconductor film. The reflectance of Cr when the wavelength is 308 nm is about 60% and the reflectance of Al for the same wavelength is about 90%. Accordingly, in irradiation of laser beam having a wavelength of 308 nm, an amorphous semiconductor region under the metal film is irradiated less than an amorphous semiconductor region that is not covered with the metal film. In other words, there is a temperature gradient between the amorphous semiconductor region under the metal film and the amorphous semiconductor region that is not covered with the metal film. Therefore crystal nuclei generated in the amorphous semiconductor region under the metal film grow laterally toward the amorphous semiconductor region that is not covered with the metal film and that remains melted. According to the report, crystal grains having a grain size of 1 to 2 μm are formed through the lateral growth.
Masakiyo Matsumura of Tokyo Institute of Technology, et al. made a presentation at the forty-seventh meeting of The Japan Society of Applied Physics and Related Societies about a method of forming a crystal grain having a large grain size at a designed position. According to the method, an organic SOG film is formed on a glass substrate and a silicon oxide film is formed on the organic SOG film. On the silicon oxide film, an amorphous silicon film is formed to bury an insulating layer (buried insulating layer) in the amorphous silicon film (FIG. 6C). The buried insulating layer is quadrangular in top view and at least one vertex of the quadrangle is 60°.
The silicon oxide film and the glass substrate form a random network of Si—O bonds. Accordingly, when the silicon oxide film is formed on the glass substrate and the silicon oxide film is irradiated with a laser beam, the energy given by the laser beam irradiation is easily transmitted to the glass substrate. However, if the silicon oxide film has a carbon-containing functional group (a silicon oxide film having a carbon-containing functional group is referred to as functional group containing silicon oxide film in this specification), the functional group terminates the bond and inhibits the film from participating in forming the network of Si—O bonds. A functional group containing silicon oxide film formed on a substrate thus has low heat transmission rate, effectively working as a heat retaining film. In this specification, having a heat transmission rate lower than that of the silicon oxide film and of the glass substrate is equal to having a heat retaining effect, and a film having the heat retaining effect is called a heat retaining film. A high heat transmission rate herein means a high heat conductivity whereas a low heat transmission rate means a low heat conductivity. In irradiating the silicon oxide film with a laser beam, a phase shift mask (FIG. 6A) is used to give a gradient in energy of the laser beam (FIG. 6B). Allegedly, the method thus form crystal grains having a large grain size at designed positions.
An article by R. Ishihara and A. Burtsev, published in AM-LCD '98, p 153-p 156, 1998, reports a laser annealing method in which a high melting point metal film is formed between a substrate and a silicon oxide film serving as a base film, an amorphous silicon film is formed above where the high melting point metal film is formed, and the substrate is irradiated with an excimer laser beam from both the front and back (the front side of a substrate is herein defined as a surface on which films are formed and the back side thereof is defined as a surface opposite to the surface on which films are formed). A laser beam applied to the front side of the substrate is absorbed by the silicon film and changed into heat. On the other hand, a laser applied to the back side of the substrate is absorbed by the high melting point metal film and changed into heat, thereby heating the high melting point metal film to a high temperature. The silicon oxide film provided between the heated high melting point metal film and the silicon film serves as a heat accumulating layer, so that the melted silicon film cools slowly. According to the report, a large crystal grain with the maximum diameter being 6.4 μm can be formed in an arbitrary place by forming the high melting point metal film in an arbitrary place.
A method called sequential lateral solidification method (the SLS method) has been developed by James S. Im of Columbia University, et al. to induce super lateral growth in a desired place. In the SLS method, a mask having a slit is moved along for every shot by a distance corresponding to the length of super lateral growth (about 0.75 μm) to crystallize the film.
The method in which a metal film is partially formed on an amorphous semiconductor film by laser beam irradiation for crystallization has drawbacks. Crystal grains obtained by this method have a small grain size of 1 to 2 μm. Also, the method can control where the crystal grains are to be formed but it cannot control the formation position on a single crystal basis. The metal film that is formed directly on the amorphous semiconductor film also can cause a problem, in that the metal elements in the film diffuse into the amorphous semiconductor film. If this amorphous semiconductor film with the diffused metal elements is crystallized to form a crystalline semiconductor film and the crystalline semiconductor film is used to form a TFT, the TFT may have degraded electric characteristics. Furthermore, the method may cause cracking or peeling in the metal film and the amorphous semiconductor film.
In the method disclosed by Matsumura et al., the phase shift mask is necessary to give gradient to laser beam energy. In order to position the phase shift mask relative to the buried insulating layer, control with a micron-level precision is needed to thereby make the laser irradiation apparatus for this method more complicated than an ordinary laser irradiation apparatus. Also, the buried insulating layer that is quadrangular in top view with one or more corners of the quadrangle having an angle as wide as 60° results in too many crystal nuclei in the semiconductor film below the wide corner or corners when the semiconductor film that has been melted by laser irradiation is cooled down. These crystal grains crowd the film and collide with one another as they grow, thereby lowering possibility of obtaining crystal grains of large grain size. Furthermore, the complicate structure of burying an insulating layer in an amorphous semiconductor film cause the trouble when it comes to forming a TFT. The trouble is that the buried insulating layer remains despite the fact that it has nothing to do with the actual function of the TFT.
The method proposed by R. Ishihara et al. can form a semiconductor film that may be used as an active layer of a top gate TFT structurally. However, this top gate TFT will have difficulty in operating at high speed because the silicon oxide film provided between the amorphous semiconductor film and the high melting point metal film generates parasitic capacitance to increase current consumption. On the other hand, the method will be useful for a bottom gate TFT or a reversed stagger TFT, for the high melting point metal film can serve as a gate electrode. Still, the method requires that a silicon oxide film is formed on a substrate, a high melting point metal film is formed on the silicon oxide film, and an amorphous silicon film is formed above where the high melting point metal film is formed. The thickness thereof, even if not counting the thickness of the semiconductor film in and considering only the thickness of the high melting point metal film and the silicon oxide film, does not amount to a thickness that is suitable both for crystallization process and for a TFT element in terms of electric characteristics. Thus the method cannot satisfy the optimal design for crystallization process and the optimal design for element structure simultaneously.
Moreover, when a high melting point metal film that does not transmit light is formed over the entire surface of a glass substrate, it cannot form a transmissive liquid crystal display device. Also, a chromium (Cr) film or a titanium (Ti) film used as the high melting point metal film has high internal stress, which probably leads to insufficient adhesion to the glass substrate. The high internal stress also influences the semiconductor film to be formed above the high melting point metal film and is likely to cause distortion in the resultant crystalline semiconductor film.
On the other hand, in order to control the threshold voltage (hereinafter referred to as Vth) that is an important parameter in TFTs so that it falls within a given range, charged electrons in a channel formation region has to be controlled. In addition, in order to obtain controlled Vth, it is required that charge defect density is reduced in a base film formed from an insulating film in contact with an active layer as well as in a gate insulating film and that the internal stress in the films is balanced. These requirements are suitably met by a material containing silicon as its ingredient, such as a silicon oxide film or a silicon oxynitride film. Therefore, there is a fear that the high melting point metal film provided between the substrate and the base film will disturb the balance.
The SLS method requires control with a micron-level precision in positioning the mask relative to the substrate, thereby making the laser irradiation apparatus for this method more complicated than an ordinary one. Moreover, the method has a problem in throughput when it is used to form a TFT for a liquid crystal display device having a large area region.